U.S. Pat. No. 5,580,443 suggests a pyrolysis process wherein fouling/coking is reduced. In this publication, a process is described for pyrolysis of a low quality feedstock into olefins by a process wherein the feed is preheated and partially vaporized in a feed preheater. The remaining liquid feed is separated at the outlet of the feed preheater in a separating device after mixing with an amount of superheated dilution steam. The amount of liquid feed to be separated is controlled by the amount and/or ratio of superheated dilution steam that is mixed upstream and downstream of the separating device. The process may make use of an economiser, without any means for controlling the capacity (heat pick-up) of the economiser.
U.S. Pat. No. 4,879,020 relates to a method of operating a furnace hydrocarbon converter. A process for thermally cracking a feed wherein the heat pick-up of a feed preheater is controlled by regulating the exchange capacity of an economiser is not disclosed in this publication either.
In U.S. Pat. No. 6,632,351, a pyrolysis process is described, wherein the feed to be separated is heated to a temperature of at least 375° C. prior to separating the feed in a liquid and a vaporous fraction.
EP-A 253 633 describes a hydrocarbon cracking furnace containing heat exchangers. Each has its own feedstock supply such that flow and pressure drop can be controlled independently. It is not suggested to control the heat pick-up of the feed preheater, and thereby the vaporisation temperature of the feedstock.
Prior art processes such as mentioned above have a limited flexibility for variations in process conditions, such as variations in feedstock characteristics, cracking severity, steam dilution ratio and furnace turndown. This is due to the fact that the separation control by mixing an amount of superheated dilution steam is only adequate for conditions that are close to a single design case. For larger deviations from the design case the amount of liquid that is separated may be too much (improper process efficiency) or too little (improper separation, causing fouling in downstream equipment).
Still, it is desired to provide alternative processes for cracking feedstock, in particular feedstock comprising a heavy tail.